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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2009–5025

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Methods

Water Quality Data Collection

Water samples were collected weekly according to established collection and quality control protocols (U.S. Geological Survey, 1997 to present). The results of the quality-assurance data associated with these samples are discussed in the appendix A. Water quality monitors were used at the time of sample collection to measure instantaneous water temperature, dissolved oxygen, pH, and specific conductance. Calibrations and data handling were performed according to techniques described by Wagner and others (2006). Finalized data are stored in the USGS National Water Information System database and can be accessed at http://waterdata.usgs.gov/nwis.

Samples were collected in two ways: (1) as a depth-integrated sample (to reduce bias of constituents that are variable with depth) for the analysis of constituents in unfiltered water, or (2) as a point sample for the analysis of constituents in filtered water. The depth-integrated samples were collected with a weighted cage that was lowered at a constant rate into the water to 0.5 m from the bottom. For the collection of samples analyzed for total phosphorus, total nitrogen, and chlorophyll a, the weighted cage was outfitted with two 1-L bottles. Each bottle had two small ports, one for water to flow in and one for the escape of displaced air. The contents of the bottles from multiple descents of the cage sampler were combined in a churn splitter. Total phosphorus and total nitrogen samples were preserved with 1 mL of 4.5 normal sulfuric acid and chilled until they were sent to the National Water Quality Laboratory (NWQL) for analysis. Chlorophyll a samples were filtered onto glass fiber filters, frozen, and sent on dry ice to NWQL. For the collection of samples analyzed for total particulate carbon, total particulate nitrogen, total inorganic carbon, and total organic carbon, the weighted cage was outfitted with a 1-L amber glass bottle. After filling, the amber bottle was capped and chilled on ice until processing. Samples were filtered using the vacuum-filtration method described by U.S. Geological Survey (1997 to present) before they were sent to NWQL for analyses.

The point samples were collected by lowering one end of a hose to mid-depth in the water column and then pumping the water through a 0.45 µm capsule filter. This filtered water was collected into a 125-mL brown polyethylene bottle that was chilled and sent to NWQL for the analysis of ammonia, orthophosphate, and nitrite-plus-nitrate. The sample for the analysis of dissolved organic carbon (DOC) was collected into a 125-mL baked amber glass bottle, preserved with 1 mL of 4.5 normal sulfuric acid, and chilled until it was sent to NWQL.

Statistical Methods

All statistical analyses were run using nonparametric methods, like the Spearman rank correlation, which was applied using Sigmaplot for Windows, version 10.0 (Systat Software, Inc., San Jose, Calif.). Analysis of variance (ANOVA) tests were applied using the Tukey studentized range test for differences in the means on rank-transformed data, and were run using SAS System for Windows™, release 9.1.3 (SAS Institute, Inc., Cary, N.C.).

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